Thermosetting powder coatings have gained considerable popularity in recent years over liquid coatings for a number of reasons. Powder coatings are virtually free of harmful fugitive organic solvents normally present in liquid coatings, and, as a result, give off little, if any, volatiles to the environment when cured. This eliminates solvent emission problems and dangers to the health of workers employed in the coating operations. Powder coatings also improve working hygiene, since they are in dry solid form with no messy liquids associated with them to adhere to workers' clothes and coating equipment. Furthermore, they are easily swept up in the event of a spill without requiring special cleaning and spill containment supplies. Another advantage is that they are 100% recyclable. Over sprayed powders are normally recycled during the coating operation and recombined with the original powder feed. This leads to very high coating efficiencies and minimal waste generation.
However, in spite of the many advantages, powder coatings traditionally have not been suited for heat sensitive substrates, such as wood and plastic articles, due to the rather high temperatures demanded to fuse and cure the powders. Recent efforts have focused on the development of powder coatings which permit polymerization or curing at lower temperatures, in order to reduce the amount of potentially damaging and deforming heat loads imposed on sensitive substrates.
Low temperature curable powder coatings based on epoxy resins have recently been proposed, as for example, as taught in U.S. Pat. No. 5,714,206. However, curing agents used to achieve the low temperature cures are based on aliphatic or aromatic amines which tend to yellow under heat. Also, epoxy coatings generally do not provide the durability and weatherability that unsaturated polyester based powder coatings normally provide.
Unsaturated polyester powder coatings are both weatherable and extremely reactive systems that undergo rapid polymerization at low temperatures, making them particularly attractive for coating heat sensitive substrates. However, one drawback with their use is that the free radical-induced polymerization or curing reaction is easily inhibited along the surface of the coating upon contact with air or, more precisely, oxygen. Atmospheric oxygen, which contacts only the surface of the coating film, while leaving the interior unaffected, adds to the terminal free radical generated on the growing addition polymer and caps it, thereby halting further polymerization and leaving the surface of the coating film uncured. The surface, therefore, remains soft and tacky and possesses inferior film properties, such as poor solvent resistance, stain resistance, and surface hardness.
Various approaches have been taken to minimize surface air inhibition. For instance, attempts have been made to incorporate polyethylene waxes in the unsaturated polyester powder formulations to provide an oxygen barrier layer on the film's surface which is eventually polished off after curing. This approach works well with liquid coatings. However, in powder coatings, the rapid curing at low temperatures does not allow the wax enough time to diffuse and rise to the surface of the film. Higher wax loadings may be used, but this tends to cause the powders to block or sinter during storage and/or to produce an aesthetically unfavorable orange peel finish when cured.
Another approach has been to incorporate oxygen-reactive species in the coatings, as for example, as taught in International (PCT) Publication WO 93/19132. Therein disclosed are low temperature curable unsaturated polyester powder coatings with a resin system composed of a mixture of unsaturated polyester resins and allyl ether curatives that is cured in the presence of free radical peroxide initiators and cobalt salt catalysts. Air inhibition is prevented by employing oxygen-reactive allyl ether curatives which consume oxygen before it can interfere with the curing reaction. However, one disadvantage of such powders is that the curatives used to get good surface cure are mostly liquids or waxy (low melting) semi-solids at room temperature. Liquid and semi-solid materials have only limited use in powder coatings. Typically, when employed beyond a few percent, they tend to cause the powders to block or sinter in storage, giving the powders poor shelf stability and making them difficult to meter and spray during coating operations. Conversion of such materials into solids is expensive and time-consuming. Another disadvantage is that the production of these low temperature curable powders is extremely difficult, since they have a tendency to pre-react and set-up in the extruder during the traditional melt-mixing process.
It would be desirable and is, therefore, a primary object of the invention to provide a melt extrudable low temperature curable unsaturated polyester powder coating suited for heat sensitive substrates that exhibits excellent surface cure without the need for waxes or oxygen-reactive curatives.